In the manufacture of carpet, tufts of carpet fabric or yarn or pile (i.e., "pile tufts") are affixed by a needle tufting process to a primary backing material such as jute or a polyolefin material which can be characterized as loosely or tightly woven. The tufts may be woven into one backing material or, as illustrated in U.S. Pat. No. 3,957,568, may be woven into a sandwich formed between two spaced, parallel primary backing materials. The "sandwich" is subsequently cut down its center to form two carpets. Adhesive may be applied to the primary backing material to hold the tufts in their woven position. The tufts are woven by the needle tufting process into the primary backing material in staggered rows to avoid lines between the tufts showing in the finished carpet. Staggering the rows results in undulations or waves of pile tufts approaching and receding the straight width edge of the carpet, i.e., the primary backing material. While the undulations for any given carpet may be somewhat periodic since the tufting process is consistent, the undulations vary in amplitude and frequency or height and length from carpet to carpet. The pile tufts are woven into the primary backing material so that an end width space is typically left between the undulating carpet pile edge and the straight edge of the primary backing material.
A secondary backing material, typically a latex secondary backing coated with an adhesive or a thermoplastic coated scrim, is then attached to the primary backing material and the carpet with its primary and secondary backing material is applied or stretched onto the pins of a tenter. Those pins typically catch the primary backing material in the width end space between the undulating carpet pile tuft edge and the width edge of the primary backing material. As the carpet is heated while it travels the length of the tenter, the adhesive dries and the backing material, especially the secondary backing material, shrinks to increase the density of the pile tufts (as explained in U.S. Pat. No. 4,579,763) while the tenter stretches the carpet to avoid carpet curl so that the carpet will lie flat when installed. The carpet edges are then trimmed to standard commercial carpet widths and wound onto rolls for transport.
It is appreciated that the stretching and drying operation performed by the tenter is critical to a high speed process forming a continuous longitudinally extending carpet. The carpet must be accurately aligned so that it can be consistently gripped by the tenter pins at the proper position to not only prevent line downtime but to also assure a properly constructed carpet. It is important then that the secondary backing material underlie the tufts of the carpet. Accordingly, the secondary backing material is supplied in widths at dimensions in excess of the pile tuft width to make sure the secondary backing material underlies the pile tufts even if the excess extends to the tenter pins gripping the primary backing material. The excess width of the secondary backing material is subsequently trimmed and discarded as scrap resulting in a more expensive carpet than what is otherwise required.
Photoelectric devices have been used to detect the straight edge position of the carpet and adjust the carpet position accordingly. U.S. Pat. No. 4,222,809 shows the use of photoelectric devices to sense opposite straight side edges of the outer covering of a foam backed carpet which are used to make adjustments to the carpet guiding mechanism to assure positioning of the opposed fabric edges of the outer covering into the tenter (the foam backing being applied in a subsequent step). The photoelectric device works in a conventional manner to determine the straight edge of the carpet's outer covering by determining the position where light passes unimpeded to a sensor. In jute backed carpets of the type to which this invention relates, it is believed similar detector devices have been used to determine the straight edge position of the primary backing material. Such arrangements assure centering of the carpet between outside edges of the primary backing material so that the carpet can be fed in an aligned manner into the tenter to minimize tenter downtime. They do not address centering of the carpet by the pile tufts so that unnecessary scraps of the secondary backing material can be alleviated nor do they tend to enhance the consistent performance of the tenter which would otherwise occur if the pile and not the carpet edges were centered in the tenter.